Discussion on Processing Technology of Stainless Steel Pipe

Due to its corrosion resistance, stainless steel pipe is widely used in pipelines of various oil routes, and the corrosion phenomenon of stainless steel pipe is analyzed. From the selection of pipes and piping connection form, this paper puts forward the stainless steel pipe in the machining process, key points of construction technology and the pipeline corrosion protection are discussed, in order to offer a reference for technical personnel.

With the rapid development of petroleum, chemical, pharmaceutical and papermaking industries, more and more chemical substances are transported by storage tanks and pipelines made of stainless steel. There are many kinds of stainless steel pipe materials, such as 304, 304L, 316L, 317, 317L, 2507 and 2205 and ferritic stainless steel, etc. due to the good corrosion resistance and processing performance of stainless steel, it is very suitable for the production of chemical pipeline. The following on the chemical transportation of stainless steel pipe processing technology and precautions to make some discussion.

Corrosion resistance and common corrosion phenomena of stainless steel pipe

The corrosion resistance of stainless steel is mainly due to a layer of oxide film called passive film, which is the cathode for the base metal. When damaged, the adjacent metal (in saline water) is used as sacrificial anode. The passivation of the passivation film depends on the chemical and metallurgical composition of the base metal. For example, the content of molybdenum (MO) is of great significance to the passivation film.

The common corrosion phenomena of stainless steel chemical pipeline are as follows:

• ① Pitting corrosion: pitting corrosion is caused by the local destruction of the passive film. It is usually caused by the presence of small external particles on the steel surface, such as welding slag and dust. It will cause holes in the metal, which can be small enough to be difficult to distinguish by the naked eye.
• ② Crevice corrosion: it is due to the existence of gaps in the structure, such as the root of welded joint, the lower end of gasket and screw, and the internal part of threaded joint. When all solutions are retained in these places, local corrosion will be formed quickly.
• ③ Stress corrosion: it occurs on materials which are often subjected to high tensile stress and are in special corrosion medium. Increasing nickel content in stainless steel can reduce the occurrence of stress corrosion cracking.
• ④ Intergranular corrosion: when some stainless steels are heated to 450 ° C ~ 850 ° C, the deposition of chromium carbide and the reduction of chromium in the vicinity of crystal interface (i.e. the chromium is reduced to less than 12%), the corrosivity of this area will be weakened, which is a serious corrosion of grain boundaries.
• ⑤ Galvanic corrosion: corrosion that occurs when stainless steel is in contact with another alloy in an electric solution, resulting in a potential difference.

The above-mentioned corrosion phenomena are usually caused by improper maintenance or use. For example, there is a high chloride content in the pipeline which becomes the most corrosive stagnant solution. If the weld of the pipe is not completely cleaned, high-temperature oxides will be produced on both sides of the root of the weld.

Selection of material and connection form of pipe joint

The selection of stainless steel pipe, flange, positioning elbow, tee, saddle foot and gasket should consider the chemical composition and element content of the material. In addition, the connection forms of stainless steel pipe are generally divided into butt welding and flange connection. Among them, flange connection is divided into ordinary flat flange connection and high foot flange connection. Generally speaking, the appearance of butt welding is more beautiful, reducing the flange joint, reducing the possibility of leakage, but it increases the difficulty for the processing side.

Treatment of pipe butt welding

The welding requirements of pipe butt welding are high, and each joint should be inspected by film. In order to reduce the oxidation corrosion in the pipe during butt welding, argon is needed to be filled for protection. If the gas is filled in the actual system, the consumption of argon is very large. In addition, in the process of construction, the passivation treatment of the pipe welded on site is relatively easy to be carried out on the external surface, while the internal part of the pipe is not so easy. A series of problems, such as passivation treatment tools and containers, large amount of waste liquid treatment and flushing water discharge, need to invest a lot of manpower and material resources, and the effect is not very ideal.

Features of flange connection

If it can be changed to flange connection, the situation will be very different. Although the use of flanges increases production costs, it also increases the possibility of leakage. However, compared with the passivation process of integral piping system, the passivation of flange connection is simple and the quality is guaranteed. The passivation treatment of flange connection only needs to apply passivation paste on the inner and outer surface of flange welding position. The process is simple and reliable, and the whole pipe system need not be treated in the later stage, only local passivation is needed. In the flange connection, the common flat flange and the high foot flange can meet the requirements of the pipeline design specification. The advantages of the ordinary flat flange connection are that the price of the flange is much cheaper than that of the high foot flange. The disadvantages are as follows: due to the different welding forms of flange, it is easy to form chemical effusion at the joint after connection, resulting in corrosion at the pipe joint.

The advantage of high foot flange connection is that there is no liquid accumulation after connection, which eliminates the shortcomings of flat flange connection. However, the price of high foot flange is higher, which increases the production cost, which is its deficiency.

Key points of stainless steel pipe processing

Blanking and chamfering of stainless steel pipe

Special cutting and chamfering machine should be used for cutting stainless steel pipe, and plasma flame cutting should not be used as far as possible. Carbon steel material should not be used in clamping support and shelf making during blanking, and stainless steel material should be used as shelf. It is better to wrap the claw of chamfering machine with a thickness of about 1.5mm. The blade of chamfering machine should be made of high-speed tool steel.

Calibration, bending and welding of stainless steel pipe

The calibration of stainless steel pipe shall be carried out on the calibration platform made of special stainless steel plate in the designated stainless steel processing area. It must be emphasized that the stainless steel should avoid contact with carbon steel in the whole production process, mainly to prevent the iron element in iron powder from polluting the stainless steel and causing crystalline corrosion. If it is inevitable to contact with carbon steel in production, it must be treated by strict passivation process.

Bending of stainless steel pipe

Stainless steel pipe manufacturing should avoid bending process as far as possible, because of the friction between the plug core of the pipe bender and the inner wall of the pipe during the bending process, the method to eliminate the pollution is to send the polluted pipe to the factory with professional passivation treatment for processing, which is neither economical nor convenient. The commonly used method in construction is to use fixed bend to make bend. In this way, the internal part of the pipe spliced after filling with argon gas will not oxidize basically. It only needs to use passivation paste on the external surface, and the workers can carry out passivation treatment by themselves to meet the quality requirements.

Welding of stainless steel pipe

To discuss the welding problem of stainless steel pipe, two problems need to be solved: one is to ensure its welding strength, which is one of the key technologies for local strength of pipeline or piping system; the other is to ensure the corrosion resistance of welding area and adjacent base metal area.

Generally speaking, the common welding method can refer to stainless steel welding process. However, different welding materials and welding process parameters should be selected according to different base metal. At the same time, attention should be paid to prevent the arc from hitting the external area of the weld; controlling the heat input and interlayer temperature to prevent overheating; preventing the weld and base metal from being excessively oxidized by shielding gas. Argon arc welding is used for welding flange and pipe, and groove single side welding and double side forming process are adopted. After welding, the welding seam shall be photographed and inspected by dye flaw detection. After passing the inspection, it shall be transferred to the next process for passivation treatment.

Connection of pipes and valves

The piping system is usually connected with butterfly valve. The butterfly valve is compact in structure, and its strength is not very high. Pay attention to the flatness and distance between the flanges during pipe calibration. Otherwise, the stress after installation is too large, coupled with the factors such as vibration during operation, it is easy to break the butterfly valve. Especially in the local repair of butterfly valve or pipe, if the pipe is too rigid and difficult to disassemble, the scope of pipeline disassembly and assembly should be expanded to avoid forced disassembly and hard installation in local area.

In the process of field welding, the butt welding of pipe should be avoided, and welding flange should be used instead to connect the pipe and pipe. The advantages of this process are as follows: first, it can avoid passivation treatment inside the pipe due to butt welding; second, it can carry out reliable passivation treatment on the inside and outside surface of the pipe by using flange connection.

In the construction process of stainless steel pipe, the fastening of horse foot can be carried out with ordinary spanner, but the wrench can not directly contact with the pipe hard collision, so as to avoid the scratch phenomenon.

Surface treatment of stainless steel pipe

Every welding seam in stainless steel pipe must be passivated. Generally, if the pipes processed in the field cannot be passivated by themselves, they shall be sent to professional manufacturers for passivation treatment. If the surface can be passivated by itself, stainless steel pickling passivation paste can be purchased for treatment. The use method of passivation paste: the passivation paste shall be evenly applied on all welding seams and grinding points with paint brush, and the surface of all processed parts shall be applied. After 24 hours, rinse with water (please operate according to the manual of passivation paste).

Requirements for processing area of stainless steel pipe

It is necessary to follow the principle that stainless steel pipes are free from contamination during storage and processing. Therefore, the following protective measures should be taken to minimize the possibility of the passive film being damaged due to contamination. When transporting stainless steel pipes, the covered lifting tools should be used. The iron rope and chuck can not directly contact the stainless steel surface, so the electromagnetic crane should not be used. The hanging basket must be tightly covered with a flat rubber of more than 3 mm. The fixing of the hanging basket and rubber shall be firmly bound with nylon tie used by electrician, and iron wire shall not be used for binding. During site installation, it should be taken from the special basket as far as possible, and can not be placed on the site at will. If it is necessary to place it on the site, non-metallic isolation materials shall be laid on the surface of the site.

Stainless steel pipes shall be stored in a separate isolation area, and shall not be mixed with carbon steel materials; the fabricated parts of stainless steel pipes shall also have a separate process storage area, and shall be far away from sandblasting parts and other carbon steel parts. When stainless steel pipes and fabricated parts are stored on steel shelves, the shelves shall be protected with wood. The small dirt on the surface should be cleaned up in time to maintain a clean and dry environment.

Discussion on surface treatment of stainless steel pipe

In order to maintain the corrosion resistance of stainless steel pipe, the surface of stainless steel pipe shall be cleaned and maintained, and the surface shall be pickled. This method is very necessary to remove the residual high temperature nitride and restore the corrosion resistance of stainless steel, and the effect is good. Before pickling, all oil stains, dirt, etc. shall be cleaned with appropriate solvent or alkaline solution, and the residue shall be washed with clean water after pickling. The other is sand blasting, which is used to remove high temperature oxides and pollutants from the surface of stainless steel. However, the sand must be clean silica sand without iron and rust particles. It should be clean new sand. Once sprayed, it can not be used for the second time.

According to the introduction of relevant construction units, the stainless steel pipes were treated with oxalic acid in series washing and passivation process. After a period of time, the pipes were corroded, but the practice proved that this method was not good, and some parts were even rotten. After inspection, the main reason is that passivation can not be carried out after welding inside the pipe. Therefore, instead of pickling process, high foot flange connection was used to eliminate the defect that the inner part of the pipe could not be passivated.

Therefore, it is suggested to adopt passivation series washing during construction, select stainless steel pump with flow rate of 3-5m3 per hour, mix the solution with nitric acid and hydrofluoric acid, and heat it to about 40°C; for 20-30 minutes of serial washing, the solution configuration shall be tested with test block.

Conclusion

To strengthen the quality monitoring of the whole process of stainless steel welded pipe processing, to achieve traceability of the quality of the processed pipe, it is necessary to monitor the construction of each process, inspect and track the name of the processor and the processing content. The specific method is as follows: make two cards of software and hardware, and record the operator, inspector, quality condition and date of each process synchronously. The contents of the project include: pipe coding, marking, cutting, chamfering, grinding, bending, pipe calibration, welding, filming, in-situ pump pressure, passivation treatment (field and field), test and acceptance. As a process flow sheet, the soft card shall be kept on completion, and the hard card shall be tied on the stainless steel pipe with copper wire or nylon rope until delivery.

Source: China Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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